As a process for producing a flat glass, a tin float process employing molten tin, a pull method, a down draw process, a fusion process and the like have been known.
However, in the above tin float process, since a glass ribbon is floated on a medium having a higher thermal conductivity than glass (on a float bath of molten tin) for forming, the transfer of heat to the medium is significant, and the influence of the temperature of the medium is significant, and accordingly the temperature control of the medium is very important. Further, gradual cooling is required under strict control by making the temperature of the medium close to the temperature of the glass so that the difference in the temperature between the surface and the inside of the glass ribbon during cooling is small. Accordingly, the cooling has to be carried out slowly, an adequately long float bath is required, and accordingly the forming time tends to be long. Further, in view of quality also, various drawbacks derived from tin are likely to occur. Further, exhaustion of tin resource is concerned.
On the other hand, in the pull method, the down draw process and the fusion process, due to forming in a vertical direction, it tends to be difficult to control the force resulting from the gravity to the glass ribbon, whereby it tends to be difficult to control the thickness of the glass, and the temperature control of the medium to decrease the difficulty becomes complicated.
Under these circumstances, the present applicant has proposed a technique relating to a process for forming a flat glass by forming a glass ribbon in a molten state into a plate shape via a thin layer of a steam film by using a base containing a steam film forming agent (hereinafter referred to as “steam float process”) (JP-A-9-295819). According to this process for producing a flat glass, such effects as resource saving, energy saving, high quality of the flat glass, decrease in equipment and operation cost, easiness of job change and possibility of variety of applications to small scale production to large scale production, can be obtained.
However, in order to further improve the smoothness of the glass surface of the formed flat glass, in addition to improvement in the forming process in a flat glass forming apparatus, improvement in the supply method capable of improving the smoothness of the glass ribbon in a molten state supplied from a glass melting furnace to a flat glass forming apparatus is required.
That is, the molten glass molten in a glass melting furnace flows as a ribbon shape glass ribbon on a lip face made of bricks at the exit of the glass melting furnace and runs down from the edge of the lip face, and lands on a flat glass forming apparatus. The surface characteristics (particularly roughness) of the lip face are likely to be transcribed to the lip face side of the glass ribbon when it flows on the lip face, and the trace of transcription may have bad influences over the smoothness of the flat glass after formed in the flat glass forming apparatus.
Further, since the viscosity of the glass ribbon which flows on the lip face is not completely the same but slightly changes, the glass ribbon is likely to run down straight down from the edge of the lip face when the viscosity is low, and the glass ribbon is likely to run down while describing a parabola from the edge of the lip face when the viscosity is high. Accordingly, the angle at which the glass ribbon runs down from the edge of the lip face is not stable, thus the glass ribbon pulsates at the edge of the lip face, and the trace of the pulsation may have bad influences over the smoothness of the flat glass. Further, because of unstable angle at which the glass ribbon runs down from the edge of the lip face, not only the landing point of the glass ribbon which lands on the flat glass forming apparatus will not be constant but also the landing impact tends to be significant when the glass ribbon runs down straight down from the edge of the lip face. Accordingly the glass ribbon is likely to be deformed by landing, and this deformation by landing may have bad influence over the smoothness of the flat glass.
The present invention has been made under these circumstances, and it is an object of the present invention to provide a process for producing a flat glass which further improves the smoothness of the flat glass after formed, by improvement in the supply method of a glass ribbon in a molten state to be supplied from a glass melting furnace to a flat glass forming apparatus.